An international survey of patients with homocystinuria due to cystathionine Beta-synthase deficiency has been completed. Data upon 629 patients have been analyzed and submitted for publication. The results obtained increases knowledge about the prevalence and time of onset of each of the major clinical manifestations of this disease: mental retardation, thromboembolic accidents, dislocated optic lenses, convulsions, osteoporosis, and early mortality. These results provide baselines adequate for design of studies of effects of therapies and for interpretation of these studies even in the absence (for ethical reasons) of control groups of patients. B6-Therapy for responsive patients or methionine restriction are demonstrated already to effectively prevent specified complications. Data on reproductive performance, effects of maternal homocystinuria, and occurrence of post-operative thromboembolic complications are also presented.